Optimal operation of multivessel batch distillation columns

Increased interest in unconventional batch distillation column configuratio ns offers new opportunities for increasing the flexibility and energy effic iency of batch distillation. One configuration of particular interest is th e multivessel column, which can be viewed as a generalization of all previo usly studied batch column configurations. A detailed dynamic model was used for comparing various optimal operating policies for a batch distillation column with two intermediate vessels. A wide variety of degrees of freedom including reflux ratios, product withdrawal rates, heat input to the reboil er, and initial feed distribution were considered. A mixture consisting of methanol, ethanol, n-propanol and n-butanol was studied using an objective function relating to the economics of the column operation. Optimizing the initial distribution of the feed among the vessels improved column performa nce significantly. For some separations, withdrawing product from the vesse ls into accumulators was better than total reflux operation in terms of ene rgy consumption. Open-loop optimal operation was also compared to a recentl y proposed feedback control strategy where the controller parameters are op timized. The energy consumption of a regular column was about twice that of a multivessel column having the same number of stages.